The site at Sonnagh is a rath, the most common type of ringfort in Ireland, essentially a circular or near-circular enclosure defined by an earthen bank and ditch that once surrounded a family's home and livestock. This particular example is subcircular in plan, measuring roughly 36 metres north to south and 32 metres east to west, and it survives in fair condition despite the ordinary agricultural pressures of the centuries.

What gives the site some of its quiet interest is the complexity still legible around its southern arc. Beyond the main scarp, there is an intervening fosse, a defensive ditch, and traces of an outer bank running from the south-south-east around through south to the south-south-west. A two-lane ringfort with a double enclosure was typically associated with a household of somewhat higher status than a single-banked example, so even these partial remains of an outer bank hint at the relative standing of whoever once lived here. A gap nearly three metres wide on the south-eastern side may be an original entrance, the point through which people and animals passed daily. Later field banks have been laid across the enclosing elements over time, partly obscuring the earlier form and folding the old settlement boundary into the working geometry of post-medieval land use.